Thread: 8.4: suppress_redundant_updates trigger vs. "Upsert" logic

Hello,

It might be helpful to point out that the "suppress_redundant_updates" trigger will cause trouble with the normal approach to doing an UPSERT, for example that specified by the "merge_db" function in example 38-2 here:
http://www.postgresql.org/docs/current/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE

If you happen to submit a redundant update against a table with the suppress_redundant_updates trigger, using the merge_db function, you'll end up in an infinite loop.

It'll similarly break any code where a result of "UPDATE 0" is assumed to indicate that the record does not exist.

Suggested changes:
1. Add to http://www.postgresql.org/docs/8.4/static/functions-trigger.html the following copy, or something nicer :)
Note that the suppress_redundant_updates trigger invalidates any logic that assumes that the number of rows affected by an UPDATE indicates the existence or non-existence of the associated rows.  Specifically, in the case of the merge_db function, any redundant updates performed by this function will cause an infinite loop.

2. Add to http://www.postgresql.org/docs/8.4/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE the following copy
Warning: Using this function or similar logic is incompatible with the suppress_redundant_updates trigger, since it is assumed that if zero rows are updated, the row does not exist.


Alternatively, the example merge_db function could be reworked to be compatible with the trigger by attempting the insert first, for example:

CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS

$$
BEGIN
    BEGIN
        -- first try to insert the row
        INSERT INTO db(a,b) VALUES (key, data);
    EXCEPTION WHEN unique_violation THEN
        -- key already existed, try updating the row
       UPDATE db SET b = data WHERE a = key;
    END;
END;
$$
LANGUAGE plpgsql;

The above is not as robust as the original, since it doesn't retry, and it's a bit of a foot-gun if you don't actually have a unique constraint...  Maybe someone else can come up with a version that's strictly better than the old one, and still plays nice with suppress_redundant_updates.

Thanks!

Mark